Archaeological Ground Point Filtering of Airborne Laser Scan Derived Point-Clouds in a Difficult Mediterranean Environment

Michael Doneus,Gottfried Mandlburger,Nives Doneus

doi:10.5334/jcaa.44

Abstract

Digital terrain models (DTM) based on airborne laser scanning (ALS) are an important source for identifying and monitoring archaeological sites and landscapes. However, a DTM is only one of many representations of a given surface. Its accuracy and quality must conform to its purpose and are a result of several considerations and decisions along the processing chain. One of the most important factors of ALS-based DTM generation is ground point filtering, i.e., the classification of the acquired point-cloud into terrain and off-terrain points. Filtering is not straightforward. The resulting DTM is usually a compromise that might show the surface below very dense vegetation while losing detail in other areas. In this paper, we show that in very complex situations (e.g., strongly varying vegetation cover), an optimal compromise is difficult to achieve, and more than one filter with different settings adapted to the varying degree of vegetation cover is necessary. For practical reasons, the results need to be combined into a single DTM. This is demonstrated using the case study of a Mediterranean landscape in Croatia, which consists of open areas (agricultural and grassland), olive plantations, as well as extremely dense and evergreen macchia vegetation. The results are the first step toward an adaptive ground point filtering strategy that might be useful far beyond the field of archaeology.

Highlights

Within the last one and a half decades, airborne laser scanning (ALS) has become a widely used technology in archaeology
Airborne laser scanning has great potential to reveal archaeological remains in a variety of contexts. This could be demonstrated in the area of the Medulin Bay on the southern extent of Istria, a typical Mediterranean environment with varying degrees of vegetation, including open areas as well as dense and mainly evergreen macchia
Testing various filters to remove the dense vegetation while keeping archaeological detail in less overgrown areas showed that there was no single filter setting that would achieve a satisfying result

Summary

Introduction

Within the last one and a half decades, airborne laser scanning (ALS) has become a widely used technology in archaeology. Even when working with project-based data that were acquired for archaeological purposes, reproducibility of the provided DTM is not possible and – even worse – its archaeological value is difficult to assess This makes it even more important to understand constraints and pitfalls that come along with ground point filtering in order to evaluate the archaeological suitability of an ALS-based DTM. To the best of our knowledge, no information can be found on archaeology-oriented ALS-point cloud classification of extremely difficult situations, including varying topography with steep slopes and disparity of vegetation density. These are situations that usually cannot be filtered with a single parameter set. This approach is demonstrated using the case study of a Mediterranean landscape in Croatia

Case study

Step 13 Classify

Result

Findings

Discussion

Conclusion